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I talked in person to Joe @ Dynospeed a couple weeks ago. Some cars are - what do you call it...factor wonders or something? Cars from the factory that are just way better than average.
He pointed me to a Mustang that he did a basic whipple HO install on, an otherwise stock car. First run on the dyno and it put down 505 rwhp when most other cars would have put down about 460. He ended up pulling it back to something more conservative.
Kenne bell claims 20 HP from electric water pump, maybe they forgot to mention some head work.
But another magazine car also claimed over 400 rwhp, although it was a stroker.